This study aims to analyse macrophage differentiation antigens and heterogeneity. We will use monoclonal antibodies against monocyte differentiation antigens to determine the quantitative expression of these antigens on normal mononuclear cells and a panel of monocytic leukemia cells. These determinations will be made using a fluorescence activated cell sorter with a logarithmic amplifier. Qualitative analysis will be made on frozen sections of lymphoid tissues from normal and pathological biopsies. A second approach aims at defining macrophage functions that are related to these antigens using in vitro blocking experiments. Special emphasis will be given to antigen-and-mitogen-induced T cells activation and lymphokine production. Finally monoclonal antibodies will be used as a probe to determine the origin of the abnormal cells in "histiocytic" disorders and lymphomas such as Hodgkin's disease.